Type synonym referred to as datatype in error message

To reproduce the issue, compile this file with GHC 8.0.1, 8.0.2, 8.2.1, or HEAD:

{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE RankNTypes #-}
module Bug where

import Data.Proxy

type Wat = forall (a :: k). Proxy a

GHCi, version 8.3.20170706: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:7:1: error:
    • Kind variable ‘k’ is implicitly bound in datatype
      ‘Wat’, but does not appear as the kind of any
      of its type variables. Perhaps you meant
      to bind it (with TypeInType) explicitly somewhere?
      Type variables with inferred kinds: (k :: *)
    • In the type declaration for ‘Wat’
  |
7 | type Wat = forall (a :: k). Proxy a
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The error message mistakenly refers to Wat as a datatype.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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